Calculating power loss for inductive power transmission

ABSTRACT

Power loss is calculated in an inductive power transfer system comprising a power transmitter for transmitting power inductively to a power receiver via transmitter coil and receiver coil. The power transmitter obtains time information for time alignment to enable the power transmitter to align the time of calculating the power loss with the power receiver. Power loss is calculated during power transfer according to the obtained time information and received power parameter communicated from the power receiver.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application a continuation of U.S. patent application Ser. No.15/484,429 filed on Apr. 11, 2017 which is a continuation of U.S. patentapplication Ser. No. 14/003,827 filed Sep. 9, 2013 which is a NationalStage application of International Application No. PCT/1132012/050905filed Mar. 12, 2012 which claims the benefit of EP Patent ApplicationNo. 11159036.0 filed Mar. 21, 2011. These applications are herebyincorporated by reference herein.

The invention relates to a method of calculating power loss in aninductive power transfer system in a power transmitter.

The invention further relates to method of enabling calculating powerloss in an inductive power transfer system in a power receiver.

The invention further relates to a power transmitter, a power receiverand a communication signal.

The invention relates to the field of power transmission technology,particularly to method and device for calculating power loss duringpower transfer.

Power transfer via magnetic induction is a well known method, mostlyapplied in transformers, having a tight coupling between primary andsecondary coil. By separating primary and secondary coil in two devices,wireless power transfer between these devices becomes possible based onthe principle of a loosely coupled transformer. The basic elements forsuch a system are a power transmitter, containing a primary coil and apower receiver, containing a secondary coil.

The document “System description, Wireless Power Transfer, Volume I: LowPower, Part 1: Interface Definition, Version 1.0 July 2010, published bythe Wireless Power Consortium” available viahttp://www.wirelesspowerconsortium.com/downloads/wireless-power-specification-part-1.html,also called the Qi wireless power specification describes wirelesstransmission of power.

To prepare and control the power transfer between a power transmitterand a power receiver in such a wireless, inductive power transfersystem, the power receiver communicates information to the powertransmitter. For example, the power receiver may communicate a datapacket indicating the received power, e.g. the rectified power.

A problem is that metal objects, positioned at the surface of the powertransmitter, can reach an undesired high temperature (higher than 65°C.) due to eddy currents caused by the magnetic field generated by thepower transmitter. This is an unwanted situation. Skin burning andplastic melting could result from this heating.

It is an object of this invention to provide a method and devices forpower transmission that allow the transmitter to avoid heating metalobjects.

For this purpose, according to a first aspect of the invention, a methodof calculating power loss in an inductive power transfer systemcomprising a power transmitter for transmitting power inductively to apower receiver via a transmitter coil and a receiver coil, comprisessteps of, by the power transmitter:

-   obtaining a received power parameter communicated from the power    receiver;-   obtaining time information for time alignment communicated from the    power receiver to enable the power transmitter to align, with the    power receiver, the time of calculating a power loss during power    transfer;-   calculating the power loss according to the obtained time    information and the received power parameter.

For this purpose, according to a further aspect of the invention, amethod of enabling calculating power loss in an inductive power transfersystem comprising a power transmitter for transmitting power inductivelyto a power receiver via a transmitter coil and a receiver coil,comprises steps of, by the power receiver:

-   communicating, to the power transmitter, time information for time    alignment to enable the power transmitter to align, with the power    receiver, the time of calculating a power loss during power    transfer;-   determining a received power parameter according to the time    information and-   communicating, to the power transmitter, the received power    parameter.

The measures have the effect that power loss is accurately determinedbecause the received power and the transmitted power are determinedaccording to the same time information, e.g. in a same, aligned, timewindow. The power loss in a metal object can be estimated by taking thedifference between the net transmitted power and the gross receivedpower. To prevent that too much power is dissipated into a metal object,the power transmitter terminates power transfer if the power lossexceeds a threshold. Advantageously metal objects are prevented to heatup by determining the power loss that is not part of the normal powerloss of the system.

For this purpose, according to a further aspect of the invention, apower receiver comprises a communication unit for communicating with apower transmitter for transmitting power inductively to the powerreceiver via a transmitter coil and a receiver coil, the power receiverbeing arranged for determining a received power parameter according totime information and the communication unit being arranged forcommunicating the received power parameter and for communicating thetime information for time alignment to enable the power transmitter toalign, with the power receiver, the time of calculating of power lossduring power transfer.

For this purpose, according to a further aspect of the invention, apower transmitter comprises a communication unit for communicating witha power receiver arranged for receiving power inductively from the powertransmitter via a transmitter coil and a receiver coil, thecommunication unit being arranged for communicating a received powerparameter and time information for time alignment, the power transmitterbeing arranged for calculating power loss during the transfer of powerfrom the power transmitter to the power receiver according to thereceived power parameter communicated from the power receiver duringpower transfer and by applying the time information communicated fromthe power receiver for the calculation of the power loss between powertransmitter and power receiver.

For this purpose, according to a further aspect of the invention, acommunication signal for communicating to a power transmitter from apower receiver arranged for receiving power inductively from the powertransmitter via a transmitter coil and a receiver coil, is arranged forcommunicating a received power parameter and time information for timealignment, to enable the power transmitter to align, with the powerreceiver, the time of calculating a power loss during power transfer forthe calculation of the power loss between power transmitter and powerreceiver.

Advantageously the devices and signal constitute a system for wirelesspower transfer, which system is enabled to calculate power loss duringthe transfer of power from the power transmitter to the power receiveraccording to the received power parameter and the time information forthe calculation of the power loss between power transmitter and powerreceiver.

Optionally, in the methods, devices and/or signal, the time informationcomprises the size of a time-window and an offset of the time-window toa time reference point.

Optionally, a method of calculating power loss in an inductive powertransfer system comprising a power transmitter for transmitting powerinductively to a power receiver via transmitter coil and receiver coil,the method comprising steps of:

-   obtaining, by power transmitter, time information for time alignment    to enable the power transmitter to align the time of calculating the    power loss with the power receiver;-   calculating power loss during power transfer according to the    obtained time information and received power parameter communicated    from the power receiver.

Optionally, the timing information comprises of the size of atime-window and its offset to a time reference point.

Optionally, the time reference point is related to the communication ofa packet from power receiver to power transmitter.

Optionally, the time reference corresponds to the end of communicating agiven bit of packet.

Optionally, the packet to which the time reference relates contains thereceived power information that the power transmitter applies tocalculate the power loss.

Optionally, the packet to which the time reference relates precedes thepacket which contains the received power information that the powertransmitter applies to calculate the power loss.

Optionally, the size of said time-window is reduced to zero or torelatively small value, whereby the power receiver takes a singlemeasurement to determine the received power and the power transmittercalculates the power loss aligned to this measurement.

Optionally, a power receiver comprises a unit for communicatingparameters related to time align parameters before power transfer toenable the power transmitter to align with the power receiver the timeof calculating of power loss during power transfer.

Optionally, a power receiver further comprises a unit for communicatingits received power to the power transmitter by:

-   a single data entity, or-   by two data entities, wherein the first data entity containing the    output value and the second entity containing information on the    power loss in the receiver, or mobile device from which the power    transmitter can calculate the received power.

Optionally, a power transmitter comprises unit for calculating powerloss during the transfer of power from the power transmitter to thepower receiver according to the received power communicated from thepower receiver during power transfer and by applying timing informationcommunicated from the power receiver before power transfer fortime-alignment for the calculation of the power loss between powertransmitter and power receiver.

Optionally, a power transmitter may also comprise:

-   a unit for applying multiple instances of received power information    communicated by the power receiver to increase the robustness of the    power loss calculation method;-   a unit for terminating the power transfer if for two or more    succeeding instances the power loss exceeds a threshold;-   a unit for taking the average of two or more succeeding instances    for calculating an average power loss over these instances.

The invention also comprises a power transfer system containing a powertransmitter as described above, and a power receiver as described above.

Further preferred embodiments of the device and method according to theinvention are given in the appended claims, disclosure of which isincorporated herein by reference.

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 shows an example of the change of the received power and thetransmitted power as a result of a load step;

FIG. 2 illustrates the definition of the time window according toembodiment; and

FIG. 3 depicts an embodiment of how to determine the time reference. Thefigures are purely diagrammatic and not drawn to scale. In the Figures,elements which correspond to elements already described have the samereference numerals.

FIG. 4 shows a method of calculating power loss, and a method ofenabling calculating power loss in an inductive power transfer system.

FIG. 5 shows a transmitter and a receiver in an inductive power system.

A method to prevent a metal object to heat up is to determine the powerloss that is not part of the normal power loss of the system. The powerloss in a metal object can be estimated by taking the difference betweenthe net transmitted power and the gross received power. To prevent thattoo much power is dissipated into a metal object, the power transmitterterminates power transfer if the power loss exceeds a threshold.

To determine the power loss the power receiver estimates its grossreceived power e.g. by measuring its rectified voltage and current,multiplying current and voltage, and adding an estimation of theinternal power losses in the power receiver. The power receivercommunicates the received power to the power transmitter, e.g. with aminimum rate such as every 5 seconds. Such minimum rate means that thedistance in time between the ends of two succeeding received powerinformation data is maximally 5 seconds.

The power transmitter estimates its net transmitted power e.g. bymeasuring the input voltage and current, multiplying the input voltageand current, and subtracting from the intermediate result an estimationof the internal power losses of the power transmitter. The powertransmitter then calculates the power loss by subtracting thecommunicated received power from the transmitted power. If thedifference exceeds a threshold, the power transmitter assumes that toomuch power is dissipated in a metal object and terminates powertransfer.

A termination Criterion is defined by:P _(T) −P _(R)>thresholdwith:

-   P_(T)=estimated net transmitted power-   P_(R)=estimated gross received power-   P_(T)−P_(R)=estimated power loss-   Threshold=safety limit

The threshold may incorporate the inaccuracy of the estimatedtransmitted power and received power. It is of importance to achievehigh accuracy in the estimation of the transmitted and received powerand to mitigate the error in the calculation of the power loss.

If the output load fluctuates over time, an error will occur in thecalculation of the power loss if the measurements and estimation of thetransmitted and received power are not aligned in time. This error canbe mitigated by taking the average of the transmitted power and receivedpower over some longer period of time.

A possible implementation of averaging the power measurement is to addmultiple samples of the instantaneous measurements and divide the resultby the number of samples over a period of time.

Another possible implementation is to take the contribution of each newsample proportionally to the previous calculated average. So for exampleif a power receiver takes 20 samples over a certain period, the newaverage would be:New Sample*1/20+Old Average*19/20.

Yet another possibility is to apply a low pass filter to damp thecontribution of the actual measured value. This could e.g. be realizedwith a capacitor that is connected to the measured signal via aresistor.

FIG. 1 shows an example of the change of the received power and thetransmitted power as a result of a load step. The period over which thepower receiver averages its received power is not known by the powertransmitter and may vary from design to design. This problem isillustrated by FIG. 1. The figure shows the change of the received power(Rx-power) and the transmitted power (Tx-power) as a result of a loadstep (a sudden increase or decrease of the load).

In FIG. 1 the window over which the average transmitted power is taken(Tx-window) is not aligned to the window over which the average receivedpower is taken (Rx-window). For example if both the Tx and Rx powerchange from 1 W to 5 W as result of a load step, the average receivedpower in the Rx-window will be 3 W and the transmitted power in theTx-window will 4 W leading to an (additional) error of 1 W in theestimation of the power loss.

As a solution, the power transmitter and power receiver could measurerespectively the average transmitted power and average received powerover the period between two succeeding received power packets. Thismethod will however fail if a received power packet does not arrive atthe power transmitter due to a communication error.

The invention reduces the error in the power loss detection method thatis caused by a dynamic output load by aligning the estimation oftransmitted power to that of the received power in time. For thispurpose, the power transmitter will obtain information for aligning thetime of measuring the transmitted power and received power. Thisinformation could be for example a time window parameter for setting atime-window to the power transmitter during the configuration phase toenable the power transmitter to align its transmitted power estimationto the received power estimation by applying this time-window.

To enable the power transmitter to eliminate, or mitigate thecalculation error of the power loss for the above described power lossmethod in case the output load fluctuates, the power transmitter needsinformation on the period in time for which the power loss has to becalculated.

For this purpose the power receiver communicates the parametersdetermining the setting for a time-window during the configuration ofthe system. The power transmitter applies the window-setting to alignthe estimation of its transmitted power to that of the received power.

There could be pre-stored a default time window in the powertransmitter. In case the power receiver does not communicate such timingparameters, the power transmitter applies default values for them.

The time window is determined by the following two parameters.

-   1. Window size—e.g. an 8 bit value. The range of this parameter    could be e.g. from 0 sec to 12.750 sec. A reasonable value for the    window size could be 1 sec. The default value could be 1 s as well.    The window size may also be reduced to a relatively small value,    e.g. 100 msec or 255 msec.-   2. Window offset—e.g. an 8 bit value. This value indicates the    offset of the time-window to a time-reference. Preferably the offset    is defined between the end of the time window and the reference    point, but it is also possible to use the start point of the time    window to determine the offset to the reference. The range of the    offset parameter could be e.g. from 0 msec to 255 msec. A reasonable    value for the offset could be 100 msec. The default value could be    100 msec as well.-   3. The time-reference can be defined according to a time a certain    bit of a packet is communicated from power receiver to power    transmitter. The packet is preferably the received power packet    which the power transmitter applies to calculate the power loss,    because by receiving this packet, the power transmitter can be sure    to have the right relation between the received power information    and the time-window over which it needs to calculate the power loss.

Example embodiments to determine the time-reference are the following:

-   a. The time-reference is determined by (the end of) the    communication of the last bit of the received power packet.    Typically the length and therefore the time to communicate the    received power packet is known by the power receiver (e.g. 20 ms)    and also the time to calculate the received power from the    measurements (e.g. 80 ms) is known by the power receiver    designer−meaning that the window offset can be determined quite    accurately (e.g. 100 ms).-   b. The time-reference is determined by the communication of the    first bit of the received power packet. This timing could be a    little more accurate since the inaccuracy of the packet transfer    time is now eliminated. The power transmitter however has to store    the time of the reception of the first bit of a packet and wait for    the reception of the remaining part of the packet before it knows    that the bit is part of a received power packet.

FIG. 2 illustrates the definition of the time window according toembodiment a. in which the reference time is defined by the end of thecommunication of the last bit of the received power packet (Rx-Power).The time between the end of the time-window and the reference time isdetermined by the window offset. The time between the start and the endof the window is determined by the window size.

A possible implementation for the power transmitter is to sample itsaverage power over small time steps and to store these values in memory.A step value could for example be 10 ms. After the reception of areceived power packet the power transmitter looks up the stored valuesand calculates the average value over the configured time-window. With awindow-size of 1000 ms and a window-offset to the last bit of thereceived power packet of 100 ms, the power transmitter has to store e.g.110 samples. The power transmitter can store the transmitted powersamples circularly whereby it overwrites the oldest sample with thenewest.

The robustness of the power loss method can be improved if the powertransmitter does not terminate the power transfer based on theinformation carried in a single power received message.

The power transmitter could wait for one, or more additional receivedpower packets before terminating the power transfer. So if thepower-loss threshold would be exceeded according to the information on asingle received power packet, the power transmitter could decide todelay the decision to terminate the power transfer to the reception of asucceeding packet.

It could terminate the power transfer if for each of two or moresucceeding received power packets, the power loss exceeds the threshold.

It could average the calculated power loss corresponding to the last twoor more received power packets and terminate the power transfer if thisaverage exceeds a threshold.

The received power can be communicated from power receiver to powertransmitter by means of a received power packet as indicated in theabove descriptions. The invention however is not limited to this form ofcommunicating the received power.

The power receiver could communicate its received power also in otherforms, like

-   Communicating (any form of) output power, like e.g. the rectified    output power and in addition information which enables the power    transmitter to calculate the received power from the output power.    The additional information could be communicated as separate packet,    but also included with the output power information in a single    packet and could for example contain:-   1. Actual power loss in the receiver which the power transmitter    should add to the output power to calculate the received power. This    information is preferably communicated within the same packet as the    output power, or in a separate packet shortly before or after an    output power packet.-   2. Power correction factor which the power transmitter should    multiply with the output power and power offset factor which the    power transmitter should add to the output power to calculate the    received power. Such correction information could be communicated by    the receiver during configuration in order to reduce the    communication overhead during power transfer.

The invention applies a window-offset towards a reference time. Theabove description makes use of the received power packet to relate suchreference time, which is later than the time-window that is applied forthe power loss calculation. Other methods to determine such referencepoint are however also possible. The following are some additionalexamples for time reference.

FIG. 3 depicts an embodiment of how to determine the time reference. Thepower receiver could send the received power packet at time equaldistances; if the power transmitter is informed about the time distancebetween these packets, it is possible to use the previous communicatedreceived power packet for the time reference. FIG. 3 illustrates how touse the previous communicated received power packet for the timereference. The last bit of the previous received power packet is thereference time. The time window starts at the window offset after thisreference time. The advantage is that the power transmitter does nothave to store the samples in order to determine its average transmittedpower from the past. It can determine its average transmitted powerduring the time window. It only has to store the resulting average powerover the time window until the next received power packet to receive. Incase the power transmitter would not receive the next received powerpacket due to a communication error, it can apply a time-out to discardthe last stored average transmitted power and use the information on theexpected time between two succeeding received power packets to startdetermining the average transmitted power for the next time window.

For this method it is needed that the power transmitter is informed onthe time distance between two succeeding received power packets. Thiscould be arranged by using a default value and/or by communicating suchvalue from power receiver to power transmitter, e.g. at theconfiguration phase of the system. In this method the power receivershould not deviate the time between communicating two succeeding packetstoo much to keep the power transmitter aligned. This could be a problemin case other control packets have to be communicated with higherpriority and cause a delay in communicating the received power packet.Hence the received power packet must have a high priority.

Another possibility is that the power receiver synchronizes to a signalfrom the power transmitter. In case the system allows communication fromPower transmitter to power receiver, e.g. by modulating the amplitude,frequency, or phase of the power signal, the power transmitter couldsend a synchronization datum at regular time intervals. Such datum couldalso function as time reference for a time window. The time window coulde.g. be exactly the time between two succeeding synchronization data.

The system applies a window over which the received power andtransmitted power are averaged. An embodiment includes the possibilityto reduce the window size to zero. This means that instead of theaverage values of received power and transmitted power, the instancevalues are taken to realize the power loss method. A practical way ofimplementation could be to apply a very small window size in which thepower receiver takes a (single) measurement to determine its receivedpower and in which the power transmitter takes a (single) measurement todetermine its transmitted power. This embodiment is less robust comparedto an embodiment in which the average of multiple measured values over atime window with larger size is taken. Robustness can however beimproved as described earlier.

In a practical embodiment the Window Offset indicates the intervalbetween the window for averaging the received power and the beginning oftransmission of the respective Received Power Packet. The value of theWindow Offset may be expressed in units by a data value in a data packetto be transferred from the power receiver to the power transmitter, e.g.3 bits. A further data value may indicate the Window Size, e.g. 5 bits.The values may be expressed in units of a few msec, e.g. 4 msec.Advantageously units of 8 msec are used, which allows for a maximumwindow size of 252 msec instead of 124 msec.

It is noted that the time window may be a relatively long period, e.g. 1sec, but also a relative short period, e.g. 64 msec. The shorter periodenables to measure the power only while no communication between Rx andTx takes place. Amplitude modulation due to communication makesmeasurements less accurate. For example, the shorter period allowsdefining the time window to be from the end of the preceding packet(e.g. the preceding received power packet) to the beginning of thecurrent received power packet, or from the end of the preceding packetto the end of the preceding packet + the window size as defined.

FIG. 4 shows a method of calculating power loss, and a method ofenabling calculating power loss in an inductive power transfer system.The inductive power transfer system has a power transmitter fortransmitting power inductively to a power receiver via a transmittercoil and a receiver coil. The method of calculating power loss startsafter the inductive power system has initiating transferring power, asshown by step INIT TR (initialize transmitting). Then the methodproceeds by performing, by the power transmitter, the following steps.The method obtains, in step OTI (obtain time information), timeinformation for time alignment communicated from the power receiver. Thetime information enables the power transmitter to align, with the powerreceiver, the time of calculating a power loss during power transfer.The method determines transmitted power data in step DTPD (determinetransmitted power data). The method obtains a received power parametercommunicated from the power receiver in step OPP (obtain powerparameter). Then the method proceeds, in step CALC (calculate), bycalculating the power loss according to the obtained time information,the determined transmitted power data and the received power parameter.

The figure also shows the method of enabling calculating power loss bythe steps performed by the power receiver. The method starts after theinductive power system has initiating transferring power, as shown bystep INIT RC (initialize receiving). The method communicates in step CTI(communicate time information), by the power receiver to the powertransmitter, time information for time alignment. The time informationenables the power transmitter to align, with the power receiver, thetime of calculating a power loss during power transfer. In step DPP(determine power parameter), a received power parameter is determinedaccording to the time information, e.g. in a time window as defined bythe time information. Then the method proceeds to communicate, in stepCPP (communicate power parameter) to the power transmitter, the receivedpower parameter.

FIG. 5 shows a transmitter and a receiver in an inductive power system.A power supply device 110, also called base station, has at least onepower transmitter 112,112 a and a system unit 115 for controlling theinductive power system. A transmitter coil 114, also called primary coilis shown connected to a transmitter power conversion unit 113, which iscoupled to a controller 111, also called a communications and controlunit. The transmitter power conversion unit 113 converts input power totransfer power to be magnetically transferred from the transmitter coilto the receiver coil. The transmitter communication unit 111 is coupledto the power conversion unit for receiving the communication signal viathe transmitter coil from the receiver coil.

A power receiver 100, usually a mobile device, has a receiver coil 104,also called secondary coil, which is shown connected to a power pick-upunit 103 which provides output power to a load 102. The power pickupunit is coupled to a receiver communication and control unit 101. Thereceiver communication and control unit is arranged for driving thereceiver coil for transmitting a communication signal via receiver coilto the transmitter coil, and is coupled to the load 102 to sense andcontrol the load power status. The inductive power transfer system asshown in FIG. 4 is based on the well known Qi standard. Thecommunication and control units in the transmitter and the receiver areadapted to perform the functions as defined above with reference toFIGS. 1, 2 and 3.

In summary, the invention proposes a method of calculating power loss inan inductive power transfer system comprising a power transmitter fortransmitting power inductively to a power receiver via transmitter coiland receiver coil, the method comprising a step of obtaining, by powertransmitter, time information for time alignment to enable the powertransmitter to align the time of calculating the power loss with thepower receiver; and a step of calculating power loss during powertransfer according to the obtained time information and received powerparameter communicated from the power receiver.

It is to be noted that the invention may be implemented in hardwareand/or software, using programmable components. Methods for implementingthe invention have the steps corresponding to the functions defined forthe system as described above.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Additionally, although a feature may appear to bedescribed in connection with particular embodiments, one skilled in theart would recognize that various features of the described embodimentsmay be combined in accordance with the invention. In the claims, theterm comprising does not exclude the presence of other elements orsteps.

Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by e.g. a single unit orprocessor. Additionally, although individual features may be included indifferent claims, these may possibly be advantageously combined, and theinclusion in different claims does not imply that a combination offeatures is not feasible and/or advantageous. Also the inclusion of afeature in one category of claims does not imply a limitation to thiscategory but rather indicates that the feature is equally applicable toother claim categories as appropriate. Furthermore, the order offeatures in the claims do not imply any specific order in which thefeatures must be worked and in particular the order of individual stepsin a method claim does not imply that the steps must be performed inthis order. Rather, the steps may be performed in any suitable order. Inaddition, singular references do not exclude a plurality. Thusreferences to “a”, “an”, “first”, “second” etc do not preclude aplurality. Reference signs in the claims are provided merely as aclarifying example shall not be construed as limiting the scope of theclaims in any way.

The invention claimed is:
 1. A method of enabling the calculation ofpower loss in a wireless power transfer system, the method comprising;communicating, by a wireless power receiver to a wireless powertransmitter, first time information indicating the length of ameasurement time window during which power transfer is to be measured,and second time information indicating a time alignment to enable thewireless power transmitter to align a measurement time window of thewireless power transmitter with a measurement time window of thewireless power receiver; measuring power received at the wireless powerreceiver during the measurement time window; and communicating to thewireless power transmitter a received power parameter indicating themeasured power received by the wireless power receiver.
 2. The method ofclaim 1, wherein the wireless power parameter is configured to beutilized by the wireless power transmitter to determine power lossduring wireless power transfer from the wireless power transmitter tothe wireless power receiver.
 3. The method of claim 1, wherein thesecond time information indicates an offset of the measurement timewindow relative to a time reference point.
 4. The method of claim 3,wherein the time reference point is associated with a data packet forcommunicating the received power parameter.
 5. The method of claim 1,wherein measuring power received comprises determining an average powerreceived during the measurement time window.
 6. A wireless powerreceiver, comprising; a wireless power receiver coil configured toreceive power wirelessly from a wireless power transmitter;communication circuitry configured to communicate to a wireless powertransmitter first time information indicating the length of ameasurement time window during which power transfer is to be measured,and second time information indicating a time alignment to enable thewireless power transmitter to align a measurement time window of thewireless power transmitter with a measurement time window of thewireless power receiver; and processing circuitry configured to measurepower received at the wireless power receiver during the measurementtime window, wherein the communication circuitry is configured tocommunicate to the wireless power transmitter a received power parameterindicating the measured power received by the wireless power receiver.7. The wireless power receiver of claim 6, wherein the wireless powerparameter is configured to be utilized by the wireless power transmitterto determine power loss during wireless power transfer from the wirelesspower transmitter to the wireless power receiver.
 8. The wireless powerreceiver of claim 6, wherein the second time information indicates anoffset of the measurement time window relative to a time referencepoint.
 9. The wireless power receiver of claim 8, wherein the timereference point is associated with the communication of a packet fromthe wireless power receiver to the wireless power transmitter.
 10. Thewireless power receiver of claim 8, wherein the time reference point isassociated with a data packet for communicating the received powerparameter.
 11. The wireless power receiver of claim 6, wherein theprocessing circuitry is configured to measure the power received bydetermining an average power received during the measurement timewindow.
 12. The wireless power receiver of claim 6, wherein theprocessing circuitry is configured to adjust the measurement of powerreceived based on an estimate of the internal power losses of thewireless power receiver.
 13. The wireless power receiver of claim 6,wherein the communication circuitry is configured to communicate thefirst time information and the second time information during aconfiguration phase of communication with the wireless powertransmitter.
 14. A power transmitter, comprising: a wireless powertransmitter coil configured to transmit power wirelessly to a wirelesspower receiver; communication circuitry configured to receive from awireless power receiver first time information indicating the length ofa measurement time window during which power transfer is to be measured,and second time information indicating a time alignment to enable thewireless power transmitter to align a measurement time window of thewireless power transmitter with a measurement time window of thewireless power receiver, wherein the communication circuitry is furtherconfigured to receive a received wireless power parameter indicating thewireless power received by the wireless power receiver during themeasurement time window; and processing circuitry configured to measurepower transmitted by the wireless power transmitter during themeasurement time window, wherein the processing circuitry is configuredto determine an indication of power loss based on the measured powertransmitted and the received wireless power parameter.
 15. The wirelesspower transmitter of claim 14, wherein the second time informationindicates an offset of the measurement time window relative to a timereference point.
 16. The wireless power transmitter of claim 15, whereinthe time reference point is associated with the communication of apacket from the wireless power receiver to the wireless powertransmitter.
 17. The wireless power transmitter of claim 15, wherein thetime reference point is associated with a data packet for communicatingthe received power parameter.
 18. The wireless power transmitter ofclaim 14, wherein the processing circuitry is configured to measure thepower received by determining an average power received during the timewindow.
 19. The wireless power transmitter of claim 14, wherein theprocessing circuitry is configured to adjust the measurement of powerreceived based on an estimate of the internal power losses of thewireless power transmitter.
 20. The wireless power transmitter of claim14, wherein the processing circuitry is configured to determine that aforeign object is present when the power loss exceeds a threshold value.